The growth of the Internet and similar local and wide area networks (LAN and metropolitan ones) based on Ethernet technology has created a potential market for telecommunications systems to be connected directly to such networks. A typical Ethernet connection to an office location with a telecommunications system is usually based on the use of half of the 8 wires of a line L separated in two unshielded twisted pair of wires giving—one pair for transmission Tx, one for reception Rx. The transmission pair is dedicated to send packets of data over the Ethernet connection; the reception pair receives packets of data over the Ethernet connection. The four other wires are usually not used for data transmission.
On the IEEE forum 802.3 (Ethernet) was set a subgroup 802.3af dealing with remote powering of data terminal equipment DTE connected to an Ethernet interface i.e. a DTE powered by a media dependent interface MDI. There is discussed the possibilities and feasibility to remotely power a terminal via center tops using two pairs of wires of the line L as conductors of a phantom circuit. Particularly in U.S. Pat. No. 6,115,468 is disclosed an Ethernet device power transmission system providing electrical power to telecommunications terminals such as Ethernet telephones and related equipment over a 4-wire Ethernet connection. This is achieved without the need for rewiring premises having an existing 4-wire Ethernet system. This system includes an input transformer, an output transformer and a pair of twisted pair conductors. The input transformer includes a pair of primaries for connection to a source of Ethernet data. The input transformer also includes a pair of secondaries, each having a center-tap. A first twisted pair conductor is connected across the first secondary, a second twisted pair conductor is connected across the second secondary and a DC-bias is provided between the respective center tops of the first and second secondaries. At the local end, the output transformer includes a first and second center-tapped primary and a first and second secondary for connection to the load device. The first and second primary center taps are connected to a power processor for extraction of DC power.
In EP1100226 is disclosed a more specific method of providing a remote power feed to a telecommunications terminal over an Ethernet connection. Previously sending a power supply current over that connection, at least one detection signal on at least two conductors of that connection is fed for detecting the presence of a telecommunications terminal adapted to receive a remote power feed. Applying such method shall prevent all risk to such terminal because the remote power feed current is sent only if the terminal has been identified as one which is adapted to receive a remote power feed. The intensity and duration of the detection signal are chosen so that the operation of detecting the terminal cannot cause any damage if it is not one which is adapted to receive a remote power fed.
The detection signal characterizing said DTE is defined by a signature usually an impedance of a predefined value connected to the two center tops of the transformers to which the two pairs of wires are connected. On the above cited IEEE forum are set values of impedance for each different power class of terminals to be connected to a LAN. In case said DTE is connected to the LAN via a RJ-45 connector i.e. via a line L made of two couples of two pairs of wires, then the some impedance is also connected to the two center taps of the transformer to which the two pairs of the other couple are connected. After the correct recognition of the power class of the DTE, the required power supply will be applied on one or both of the two couples of two pairs of wires used possibly as conductors of a phantom circuit. Moreover, the DTE contains two detectors each one connected at the two center taps corresponding to a couple of two pairs of wires. These detectors measure the corresponding voltage present at the respective two center taps. When such measure exceeds a predefined value then they activate a respective switch letting said power supply remotely powering said DTE.
A problem will appear when the required power for a specific terminal can not be supplied by any Ethernet equipment coupled to said DTE via said line. This may occur when the power class of the DTE will exceed the maximum power which can be supplied by the corresponding Ethernet equipment. In that case, no power at all will be supplied to this DTE. And the user of such DTE will not be aware about the origin of the power supply problem. He/she may believe of a breakdown of its terminal or the line.